Synergistic suppression effects of flame retardant, porous minerals and nitrogen on premixed methane/air explosion

Highlights

• The suppression performance of N₂-powder inhibitor was investigated.

• Thermal decomposition and gaseous products of powder inhibitor were studied.

• Inhibition mechanisms of N₂-powder inhibitor were ascertained.

Abstract

A novel composite inhibitor based on porous mineral materials and conventional flame retardant of ammonium polyphosphate (APP) is prepared to suppress the premixed methane/air explosion. Taking advantages of gas and powder inhibitor, N₂ and the prepared composite inhibitor are combined to use. The suppression performance of N₂-composite inhibitor on methane explosion is investigated on a 20-L spherical experimental explosion apparatus and the characteristic pressure data are obtained. The combined inhibition effects of N₂ and the prepared composite inhibitor are greater than either acting alone. Thermal decomposition behavior and gaseous products of composite inhibitor are analyzed with thermogravimetric analysis and thermogravimetric-mass spectrometry, respectively. Based on physical and chemical actions, the inhibition mechanisms of N₂-composite inhibitor system are proposed. This work provides a reference to prepare high-performance gas explosion inhibitor based on the synergism of binary or multiple components.

Introduction

Methane gas explosion is one of the most serious disasters in underground coal mining and causes huge property loss and serious casualties. Furthermore, the hazard of gas explosion widely exists in the transportation and utilization processes. To reduce or eliminate the explosion hazard of methane, the development of new and efficient suppression technologies is highly desirable. The inhibitors play an important role in the suppression or mitigation of gas explosion. The explosive chain reactions may be completely terminated by these inhibitors, and then the intensity of gas explosion can be reduced.

Great efforts have been made to develop high-performance inhibitors. Various gas (such as N₂ and CO₂), liquid (such as water mist) and solid inhibitors have been employed (Du et al., 2014; Jiang et al., 2016; Liang et al., 2013; Pei et al., 2016; Wang et al., 2012; Wang et al., 2014; Xu et al., 2013; Zhang, B. et al., 2014a, 2014b). N₂ and CO₂ are two most widely used gas inhibitors (Wang et al., 2014). The inerting effects and mechanisms of N₂ and CO₂ on gas explosion have been explored. The maximum explosion pressure (P_max) gradually decreases with increasing the concentration of incombustible gas (Zhang, B. et al., 2014a, 2014b). N₂ or CO₂ exerts great influence on the explosion limit, limiting oxygen concentration and explosion intensity of methane (Jiang et al., 2016). Mitu et al. found that N₂ and CO₂ are more efficient than He and Ar (Mitu et al., 2016). Furthermore, ignition delay phenomenon of methane is observed after dilution with N₂ and CO₂. The dilution effect of gaseous additives can decrease the concentration of O₂, and these molecules of diluents may interrupt the chain radical reactions by physical collision with reactive radicals (Pei et al., 2016). Thus, the reactions between methane and O₂ are effectively suppressed. Furthermore, NH₃ has attracted considerable attention in inhibition of gas explosion (Luo et al., 2017). Luo et al. found that NH₃ can diminish the explosion limit range and mitigate the explosion risk (Luo et al., 2017). NH₃ possesses a strong tendency to consume HO·, H· and oxygen, thus the radical chain reactions of methane combustion or explosion are greatly retarded (Luo et al., 2017).

Due to the features of eco-friendly and abundant, water mist has been employed for gas explosion suppression (Cao et al., 2015; Yu et al., 2016; Zhang, P. et al., 2014a, 2014b). The widely-acknowledged suppression mechanisms of water mist are heat absorption, blocking thermal radiation and suffocation effects (Cao et al., 2015; Yu et al., 2016; Zhang, P. et al., 2014a, 2014b). Xu et al. found that when the volume flux of water mist is sufficiently large, the explosion of methane/coal dust mixture is completely mitigated (Xu et al., 2013). In order to improve the suppression effects of water mist, some additives, such as alkali salts and NH₄H₂PO₄, are incorporated in the water (Cao et al., 2015; Yu et al., 2016). Cao et al. investigated the suppression effect of water mist containing NaCl on methane/air explosion (Cao et al., 2015). The results indicated that the addition of NaCl can improve the suppression performances of pure water mist. The improved performance is attributed to the aforementioned physical effects of water mist and chemical inhibition effects of NaCl on combustion reactions (Cao et al., 2015). However, the applicability of water mist is limited under both high and low ambient temperature.

Due to the advantages of easy storage and transportation, various powder materials have been employed in the field of explosion suppression. Rock dust is calcium carbonate powder and has been used for about 100 years to inhibit flame propagation and coal dust explosion (Huang and Honaker, 2016a, 2016b; Song et al., 2018; Song and Zhang, 2018). The suppression performance of rock dust is influenced by particle size, dosage and dispersibility. However, rock dust suffers from adverse caking issue, leading to the deteriorated dispersibility (Huang and Honaker, 2016a, 2016b). The physical or (and) chemical suppression explosion effects of NaHCO₃, SiO₂, Al(OH)₃ and NH₄H₂PO₄ have been reported in the literature (Chelliah et al., 2003; Wang et al., 2012, 2017; Wen et al., 2018). Liu et al. studied the suppression effects of NH₄H₂PO₄, SiO₂ and rock dust on methane/coal dust/air explosions (Liu et al., 2013). The results indicated that overpressure and propagating velocity of explosion waves are decreased by these suppression agents. NH₄H₂PO₄ exhibits physical effect of endothermic decomposition reaction, which decreases the system temperature. The decomposition products from NH₄H₂PO₄ can interrupt the chain reactions of methane explosion (Jiang et al., 2016; Luo et al., 2014). Among powder inhibitors, porous materials have attracted extensive attention. The porous structure with labyrinth effect and large surface area provides abundant contact area, thus the radicals are readily quenched, and then flame or explosion is inhibited (Luo et al., 2017; Wang et al., 2017). Wang et al. prepared NaHCO₃/red mud with abundant micropores and investigated its suppression effect on methane explosion (Wang et al., 2017). The free radicals are trapped by the micropores in red mud, and explosion chain reactions are retarded.

In order to obtain high-performance inhibitor, multi-component powder inhibitors are prepared by achieving the synergism. The multi-component inhibitors exhibit better performance than single components (Jiang et al., 2016; Luo et al., 2014; Pei et al., 2016; Sun et al., 2019; Wang et al., 2014, 2017). Sun et al. prepared ammonium polyphosphate (APP)-aluminium hydroxide-kaolinite composite inhibitor, which can significantly suppress gas explosion (Sun et al., 2019). Wang et al. studied the suppression effect of NaHCO₃/red mud on gas explosion (Wang et al., 2017). After loading of NaHCO₃, red mud exhibits enhanced explosion suppression. Furthermore, gas and powder inhibitors have been combined to improve gas explosion suppression. Jiang et al. found that the combination of N₂ and ABC powder has an improved suppression effect on explosion overpressure and flame propagation speed (Jiang et al., 2016).

In order to obtain excellent suppression performance, composite inhibitor with porous materials and flame retardant was prepared. Then solid-gas suppression system was designed based on this composite inhibitor and incombustible gas of N₂. The suppression performances were studied in a 20-L spherical experimental explosion apparatus. The inhibition mechanisms were concluded by analyzing the possible synergism between N₂ and solid-phase materials.